Semiconductor field effect devices (e.g., field effect transistors) or semiconductor junction-based charge control devices (e.g., diodes, thyristors, bipolar junction transistors, etc.) must be designed with careful consideration and control of the electric fields in specific regions of the device volume during operation. For example, a key parameter for a semiconductor device of given physical device dimensions is known as the breakdown voltage (Vbr). This voltage is taken between the two terminals through which all or most of the electric current flows, such as between the source and drain. Voltage across these terminals can cause the electric field in certain areas of the active device volume to exceed a material-dependent value (known as the critical field Ecrit). When Ecrit is exceeded, excessive current flows through the device. Hence, this maximum voltage where excessive current flows (Vbr) depends directly on the maximum electric fields attained in the active device volume at a given terminal voltage.
Theoretically, modern high voltage transistors have the potential to be developed as a power switching device for some of the highest voltage applications. For example, the material properties of GaN and AlN, which are generally employed in high electron mobility transistors (HEMTs), should allow for high voltage power switching. In reality, non-uniform charge distribution and the HEMT architecture prevent this from being realized. The non-uniform electric field distribution leads to the critical field Ecrit being exceeded for voltages much smaller than predicted by the unipolar critical field. Once the critical field is exceeded, no further improvement in Vbr can be realized by increasing the gate to drain spacing.
To address these issues in non-uniform electric field distribution, modern high voltage transistors typically use a field plate to decrease the maximum electric field Ecrit attained in the device volume for a given set of terminal voltages and device dimensions, and hence increase Vbr for given device dimensions. The traditional field plate is made from a conductive material with a minimal change in potential over the length of the field plate. Such field plates generally vary in physical structural features (e.g., vary in field plate geometry, the number of field plates, or the spacing between two or more plates) and, thus, can be limited by physical scalability issues. For instance, the horizontal spacing between field plates can be controlled to enhance field sharing (e.g., by decreasing the horizontal spacing) but at the cost of reducing electric field peaks near the gate. Vertical spacing of field plates can result in a more uniform electric field but must be accommodated within a small height from the device surface. Thus, additional apparatuses are needed that have improved control over electric field distribution, especially for high voltage devices and applications. In particular, apparatuses that are not limited by physical structural features of field plates would be greatly beneficial.